In accordance with recent developments in various information devices, storage capacities of magnetic recording media have increased steadily. Particularly, magnetic disks that play a central role in the external memories of computers are increasing in both recording capacity and recording density year after year, and development of still higher-density recording has been demanded. For example, in accordance with development of laptop personal computers and palmtop personal computers, recording units that are small in size and withstand shocks have been demanded, and therefore, magnetic recording media that realize high-density recording and have high mechanical strength have been demanded. Furthermore, recently, for navigation systems and portable music reproducers, microminiature magnetic recording media have been employed.
Conventionally, as substrates for magnetic disks as magnetic recording media, an aluminum alloy substrate, a substrate formed by plating NiP on the surface of an aluminum alloy substrate, and a glass substrate have been used. However, the aluminum alloy substrate is poor in abrasion resistance and machinability, and in order to cover these defects, NiP plating is applied, but, the substrate subjected to this NiP plating has a problem in that it easily warps and becomes magnetic when it is treated at a high temperature. In addition, the glass substrate has a problem in that a deformed layer is generated and a compressive stress acts on the surface when it is reinforced, and the glass substrate easily warps when it is heated.
In the case of a microminiature magnetic recording medium with a diameter of 1 inch (27.6 mm) or 0.85 inches (21.6 mm) enabling high-density recording, substrate warp is a fatal defect. As a substrate of a microminiature magnetic recording medium, a material that is as thin as possible and does not substantially deform under external stress, and has a smooth surface for easily forming a magnetic recording layer is desirable.
Therefore, using a silicon, that has been frequently used as a semiconductor device substrate, as a substrate of a magnetic recording medium, has been proposed (for example, refer to Patent Document 1).
Monocrystalline silicon has many advantages in that it is lower in specific gravity, higher in Young's modulus, lower in coefficient of thermal expansion, and higher in high-temperature performance than aluminum, and has conductivity, and monocrystalline silicon is preferable as a substrate material for a magnetic recording medium. The smaller the diameter of the substrate, the less the shock the substrate receives, and even when a silicon substrate is used, a durable magnetic recording device is realized.
Normally, to manufacture a substrate for a magnetic recording medium, first, a monocrystalline silicon ingot is formed by a pulling method. Next, a circular through hole is fabricated at the center, and then the ingot is sliced to a predetermined thickness.
The sliced donut-shaped disk is chamfered at the edges of the central circular hole and the outer circumference with a grinding stone, and then lapping or polishing is applied to both surfaces and the surfaces are mirror-finished, and then the disk is used.
During the manufacturing process described above, the silicon substrate is transported by being housed in a transporting processing cassette, but, the material of the silicon substrate is brittle, and cracking and chipping easily occur. If the silicon substrate cracks or is chipped, this causes not only lowering in the production yield of the magnetic recording medium but also errors during recording or reproduction or crash of the magnetic head during recording or reproduction due to particles generated by rubbing against the processing cassette.
In order to obtain a substrate for a magnetic recording medium crack free and chip from a brittle material such as silicon, a method of machining ed lengths of 0.03 mm or more and 0.15 mm or less while the chamfering angles at the inner circumference and the outer circumference of the substrate are set to 20 degrees or more and 24 degrees or less has been proposed (for example, refer to Patent Document 2).
FIG. 5 shows a longitudinal sectional view of a conventional silicon substrate for a magnetic recording medium. In FIG. 5, between the main surfaces 2 and 3 of the substrate 1 and the end face 4, chamfered portions sloped at an angle α of 20 degrees or more and 24 degrees or less are provided. The lengths L of these chamfered portions are set to 0.03 mm or more and 0.15 mm or less. The same chamfered portions are also provided on the substrate inner circumferential portion although these are not shown in the figure. In this way, by using a substrate having such an outer circumferential portion shape, defects such as flaws and chipping, etc., of the substrate due to handling or dropping during the manufacturing process are reduced, and the production yield is remarkably improved.
In addition, in a glass substrate, in order to realize high-density recording, low-floating of the magnetic head with respect to the magnetic recording medium has been attempted, and recording and reproducing methods have gradually shifted from the contact start stop (CSS) method to the load/unload method (ramp loading method). In these recording and reproducing methods, a substrate with high loading reliability without errors during recording or reproduction and without crash of the magnetic head during recording or reproduction has also been demanded.
As a substrate that meets this demand, a substrate having curved portions with a radius of 0.003 mm or more and less than 0.2 mm interposed at least either between the end faces and the chamfered portions of the substrate or between the main surfaces and the chamfered portions of the substrate has been proposed (for example, refer to Patent Document 3).
A magnetic recording medium with high loading reliability without errors during recording or reproduction and without crash of the magnetic head during recording or reproduction is obtained by using this substrate.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H06-76282
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H07-249223
Patent Document 3: Japanese Unexamined Patent Application, First